What is the best way to test if a CharacterSet contains a Character in Swift 4?

Swift 4.2 CharacterSet extension function to check whether it contains Character:

extension CharacterSet {
    func containsUnicodeScalars(of character: Character) -> Bool {
        return character.unicodeScalars.allSatisfy(contains(_:))
    }
}

Usage example:

CharacterSet.decimalDigits.containsUnicodeScalars(of: "3") // true
CharacterSet.decimalDigits.containsUnicodeScalars(of: "a") // false

Not sure if it's the most efficient way but you can create a new CharSet and check if they are sub/super-sets (Set comparison is rather quick)

let newSet = CharacterSet(charactersIn: "a")
// let newSet = CharacterSet(charactersIn: "\(character)")
print(newSet.isSubset(of: CharacterSet.decimalDigits)) // false
print(newSet.isSubset(of: CharacterSet.alphanumerics)) // true